This invention relates to heat exchanger/evaporators, and more specifically, to hydrophilic surfaces employed in heat exchangers to provide improved evaporation. It also relates to compositions for making hydrophilic surfaces and to methods of making a heat exchanger/evaporator.
Evaporators come in many types and sizes. In one type of evaporator, a first heat exchange fluid is brought into heat transfer relation with a liquid to be vaporized into a gaseous stream. This type of heat exchanger may be used for humidification purposes where a humidified gas, including air, is required. By way of example only, one instance of the need for a humidifier of this type is in PEM type fuel cell systems. In many such systems, a hydrogen rich gas along with an oxygen rich gas are provided to a fuel cell with membranes separating the anode and cathode sides. Optimal efficiency of operation requires that the fuel and the oxidant therefor be delivered at or above a certain temperature. It is also required that the fuel and oxidant be delivered at a particular relative humidity so as to avoid damage to the membranes as, for example, by drying out.
Thus, heat exchangers of this type are required to evaporate an aqueous material to achieve a desired humidity level in the gaseous stream constituting the hydrogen rich stream and/or the oxygen rich stream. They may also be called upon to elevate the temperature of the streams so that optimal fuel cell efficiency results.
In many instances, particularly in fuel cell systems where size and weight are of concern, it is desirable that the heat exchanger/evaporator be of minimal size and weight. This is true, for example, in vehicular applications of fuel cell systems for traction purposes. It is difficult, however, in many situations to minimize the size of the heat exchanger/evaporator without sacrificing efficiency of humidification or uniformity of humidification.
The present invention is directed to overcoming one or more of the above problems.
It is a principal object of the invention to provide a new and improved heat exchanger/evaporator for evaporating a liquid, particularly but not necessarily an aqueous liquid, into a gaseous fluid. It is also a principal object of the invention to provide a composition for use in forming a hydrophilic surface for disposition on an evaporative heat transfer surface. It is still a further principal object of the invention to provide a new and improved method of making a heat exchanger that includes an evaporative heat transfer surface.
According to a first facet of the invention, a heat exchanger/evaporator made according to the invention includes a thermally conductive element separating a first flow path for a first heat exchange fluid and a second flow path for a second heat exchange fluid that is typically a gas. A first surface is located on the element in heat transfer relation with the first flow path and a second surface is located on the element opposite the first surface and in heat exchange relation with the second flow path. A hydrophilic coating is bonded on at least part of the second surface and is made up of a powder of nominally spherically shaped particles including nickel, chromium, aluminum, cobalt and yttrium oxide bonded together with a braze metal predominantly made up of nickel, chromium and silicon and diffused into the nominally spherically shaped particles and the second surface to bond them together. The weight ratio of nominally spherically shaped particles to braze metal is in the range on the order of 2-3 to 1.
In a preferred embodiment, the weight ratio is approximately 70:30.
In a preferred embodiment, the element is an imperforate element and has a fin bonded thereto opposite the first surface. The second surface carrying the hydrophilic material is located on the fin.
According to another facet of the invention, a composition for use in forming a hydrophilic surface for disposition on an evaporative heat transfer surface is provided. The composition includes a mixture of a powder of nominally spherically shaped particles including nickel, chromium, aluminum, cobalt and yttrium oxide together with a braze metal powder predominantly made up of nickel, chromium and silicon. The weight ratio of the nominally spherically shaped particles to the braze metal powder is in a range on the order 2-3 to 1. Also included in the composition is a volatizable organic binder that volatizes at temperatures that are sufficiently high to melt the braze metal powder and which will leave substantially no residue.
In a preferred embodiment, the binder is acrylic or polypropylene carbonate based.
According to still another facet of the invention, there is provided a method of making a heat exchanger including an evaporative heat transfer surface and which includes the steps including a step of (a) assembling a heat exchanger core assembly having at least two flow paths, a first for a first heat exchange fluid and a second for a gaseous second heat exchange fluid into which a liquid is to be evaporated. The core assembly includes plural metal components in abutting but unjoined relation. Prior to or after the performance of step (a), the method includes the step of (b) coating at least one component fronting on the second flow path with a composition including a powder of nominally spherically shaped particles including nickel, chromium, aluminum, cobalt and yttrium oxide, a braze metal powder predominantly made up of nickel, chromium and silicon and a volatizable organic binder that volatizes at temperatures sufficiently high to melt the braze metal powder and leave substantially no residue. The weight ratio of the nominally spherically shaped particles to braze metal powder is in a range on the order 2-3 to 1. A further step includes (c) subjecting the core to an elevated brazing temperature to (i) melt the braze metal and cause it to diffuse into the nominally spherically shaped particles and the at least one metal component, (ii) volatize the binder and eliminate substantially all residue thereof, and (iii) braze the metal components into a bonded assembly.
Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings.